Sequelize One-to-Many Association example with Node.js & MySQL

Sequelize One-to-Many example Overview

In systems analysis, a one-to-many relationship refers to the relationship between two entities A and B in which an element of A may be linked to many elements of B, but a member of B is linked to only one element of A.

For example, assume that you want to design a Tutorial Blog data model, you can think about one Tutorial has many Comments, but one Comment only belongs to one Tutorial.

So the relationship between Tutorial entity and Comment entity is one-to-many. That’s what we’re gonna make in this article, and here are the step by step:

  • First, we setup Node.js App

  • Next, configure MySQL database & Sequelize

  • Define the Sequelize Model and initialize Sequelize

  • Then we create the Controller for creating and retrieving Entities

  • Finally we run the app to check the result

Use Sequelize for One-to-Many Association

We’re gonna define the models using Sequelize Model:

const Tutorial = sequelize.define("tutorial", { ... })
const Comment = sequelize.define("comment", { ... })

Tutorial.hasMany(Comment, { as: "comments" });
Comment.belongsTo(Tutorial, {
  foreignKey: "tutorialId",
  as: "tutorial",
});

Using Model.create() for creating new objects:

Then show the tutorial with comments inside using Model.findByPk() and Model.findAll():

The result will look like this:

Sequelize One-to-Many Implementation

Create Node.js App

First, we create a folder:

Next, we initialize the Node.js App with a package.json file:

We need to install necessary modules: sequelize, mysql2. Run the command:

The package.json file should look like this:

Let’s create Node.js project structure like following directory tree:

sequelize-one-to-many-node-example-project-structure

Configure Sequelize and MySQL database

In the app folder, we create a separate config folder with db.config.js file like this:

First five parameters are for MySQL connection. pool is optional, it will be used for Sequelize connection pool configuration:

  • max: maximum number of connection in pool

  • min: minimum number of connection in pool

  • idle: maximum time, in milliseconds, that a connection can be idle before being released

  • acquire: maximum time, in milliseconds, that pool will try to get connection before throwing error

For more details, please visit API Reference for the Sequelize constructor.

Define the Sequelize Model

In models folder, create tutorial.model.js file like this:

This Sequelize Model represents tutorials table in MySQL database. These columns will be generated automatically: id, title, description, createdAt, updatedAt.

After initializing Sequelize, we don’t need to write CRUD functions, Sequelize supports all of them:

We’re gonna use Sequelize Model functions in our Controller.

Now continue create Comment model in comment.model.js:

Initialize Sequelize

Let’s initialize Sequelize in app/models folder. Now create app/models/index.js with the following code:

We use hasMany() to help one Tutorial have many Comments, and belongsTo() to indicate that one Comment only belongs to one Tutorial.

Create the Controller

Inside app/controllers folder, let’s create tutorial.controller.js and export these functions:

  • createTutorial

  • createComment

  • findTutorialById

  • findCommentById

  • findAll

First we need to import database object:

Create and Save new Tutorials

Create and Save new Comments

Get the comments for a given tutorial

Get the comments for a given comment id

Get all Tutorials include comments

Check the result

Open server.js and write the following code:

– First, we import our database object and controller above. – Then we call Sequelize sync() method.

In development, you may need to drop existing tables and re-sync database. Just use force: true as following code:

Now, we’re gonna test the result by putting the next lines of code inside run() function. You can run the this Sequelize one-to-many Node.js application with command: node server.js.

Create Tutorials

Create Comments

Get Tutorial by given id

Get Comment by given id

Get all Tutorials

Check MySQL Database:

– tutorials table:

sequelize-one-to-many-node-example-table-1

– comments table:

sequelize-one-to-many-node-example-table-2
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